Build and publish a docker base image for checkers

[ldruschk]

Most checkers use identical gunicorn configs and have no other dependencies than the ones imposed by gunicorn and enochecker itself. Therefore it might be a good idea to provide a base image which allows us to do under-the-hood changes without the need to change the configuration of each individual checker.

If we do this we would have to make sure that we don't do backwards incompatible changes before ENOWARS4 to not break any checkers using the base image. With the resulting Image the only thing needed besides the checker.py would be the following Dockerfile:

FROM enoflag/enochecker:latest
ADD checker.py /checker/

In this case you could even use a bind mount in docker-compose without requiring a Dockerfile at all.

If the checker requires dependencies beyond the ones from enochecker/gunicorn, it could look something like this:

FROM enoflag/enochecker:latest
ADD requirements.txt .
RUN pip install -r requirements.txt
ADD checker.py /checker/